Tunnel kiln for the heat treatment of products having a circular cross-section

ABSTRACT

Tunnel kiln for the heat treatment of products having a circular cross-section, such as bars, tubes, comprising means for making the products circulate in the kiln perpendicularly to their longitudinal axis and means for subjecting them at the same time to a rotation about that longitudinal axis, by rolling on the cylindrical surfaces. The cylindrical surfaces are fast individually or in groups with at least one structure subjected to a linear movement along the longitudinal axis of the kiln.

United States Patent Weber et a1. June 3, 1975 [54] TUNNEL KILN FOR THE HEAT 2,372,913 3/1968 Gaabs .1 432/124 TREATMENT OF PRODUCTS HAVING A 2,707,629 4/1955 Kennedy 432/124 CIRCULAR CROSSSECTION 2,795,411 6/1957 Court 432/124 [75] Inventors: Jean Weber, Tarbes; Jean Moussou,

gagneres De BigOI-re both f Primary Exam1nerJohn J. Camby France Attorney, Agent, or FirmSughrue, Rothwell, Mion, Zinn & Macpeak [73] Assigneez Ceraver, Paris, France [22] Filed: Apr. 25, 1974 21 Appl. No.: 464,091 [57] ABSTRACT Tunnel kiln for the heat treatment of products having [30] Foreign Application Priority Data a circular cross-section, such as bars, tubes, compris- May 8, 1973 France 73.16587 1118 means for making Products circulate in the kiln perpendicularly to their longitudinal axis and 52 vs. C] 432/124; 432/138 means for subjecting them Same time a rota- 51 1111. C1. F271) 9/24 about that longitudinal axis by filling the W- 5 Field of Search II I u 432/12 122 124 137 lindrical surfaces. The cylindrical surfaces are fast in- 432/138 dividually or in groups with at least one structure subjected to a linear movement along the longitudinal [56] References Cited axis of the klln' UNITED STATES PATENTS 13 Claims, 18 Drawing Figures 1,551.827 9/1925 Ladd 432/124 1O 27 g E 1 Q a O 25 i FATE? WFRJUFI 3 ms SHEET NOE RPJEHTFQJUIS 19?; 3 8 1327 o O o a cg o F 0 o o U 066 o 3 %8y 5 00006 llllii PMEE :TFHJIBE 3 m5 SHEET FIG.11

FlGflZ rllwllllulllll ill TUNNEL KILN FOR THE HEAT TREATMENT OF PRODUCTS HAVING A CIRCULAR CROSS-SECTION BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention concerns a tunnel kiln for heat treatment of products having a circular cross-section, comprising means for making the products circulate in the kiln perpendicular to their longitudinal axis and means for subjecting them at the same time to a rotation about that longitudinal axis by rolling on cylindrical surfaces. It applies more particularly to the firing of elongated cylindrical parts such as bars, tubes, which must not undergo, during that firing, any deformation due to their weight or to non-homogenous heating effects, and more particularly to the firing of ceramic parts. It is suitable however for other applications such as heat treatment of metallic parts.

2. Description of the Prior Art It has already been proposed, to avoid any deformation of the products subjected to the heat treatment in a kiln, to arrange these products in tubular cylindrical supports rotating on their axis during the firing. Kilns comprising a tube crossing the chamber of the kiln, rotating on its axis, driven by sprockets or gears, in which the product to be fired is inserted have thus been produced. The tube may be or not be stopped up at one end. In the first case, the product to be fired is inserted and withdrawn on the same side; in the second case, the product may be inserted in a continuous way on one side and withdrawn from the other.

It has been proposed also to substitute the support tube by a drum rotating about its axis and comprising several support tubes. In these various embodiments, the axis of rotation of the support tube or of the drum is fixed in the kiln.

It has also been proposed to arrange the products to be fired in tubular supports arranged in a layer on the inclined hearth of a continuous kiln, parallel and perpendicular to the axis of the kiln. By controlling the retaining of the layer, it is possible to obtain a movement of the latter by the rolling of each support tube which therefore rotates about its axis.

It has also been proposed to arrange the products to be fired in several superimposed layers resting on rollers supported by carriages moving on rails parallel to the longitudinal axis of the kiln, the rollers being set in rotation by means of sprockets rolling on lateral roller paths.

The output of such kilns is relatively limited; the multiplying of the number of layers of support tubes cannot be considered without complications. It then oecomes compulsory, indeed, to ensure a correct rolling of the tubes, to observe fairly delicate conditions, depending on the density of the support, the friction coefficients, the slope of the hearth, the number of layers, the length of the kiln, etc.

An object of the present invention is to enable the simultaneous heat treatment of a great number of products having a circular cross-section, in a tunnel kiln having a length which remains acceptable. A secondary object is to enable, if need be, the imparting, to the cylindrical rolling surfaces, of a variable circumferential speed according to the temperature level in the zone of ie kiln through which they cross, in order to avoid any deforming of the products, even in the zone having the highest temperature level in the kiln.

SUMMARY OF THE INVENTION A tunnel kiln according to the invention is characterized in that the cylindrical surfaces ensuring the rolling of the products are grouped on at least one drum progressing by rolling along the kiln.

It comprises, moreover, preferably at least one of the following characteristics:

it comprises two lateral roller paths and the drum is provided at each of its ends with roller sprockets on these roller paths having diameters different from its own;

the drum is provided with at least two circular flanges resting on the hearth of the kiln;

it comprises at least two drums and a roller provided with guide trunnions mounted idle on its axle is arranged between two successive drums;

the drums are provided with sprockets and the diameter of the guide trunnions of the roller is smaller than that of the sprockets of the drums;

in a kiln comprising at least two drums provided with guide sprockets, a circular distance piece rolling by its periphery against the said sprockets and provided with two guide sprockets mounted idle on its axle is arranged between two successive drums;

in a kiln comprising at least two drums provided with guide sprockets, a distance piece consisting of two flanges resting on the axles of an even number of sprockets, having the same diameter and rolling on the same roller paths as the sprockets of the drums, connected together and to the sprockets of the ,drums through the odd number of sprockets immediately greater than the above even number, having axles at a lower level than that of the axles of the preceding sprockets and having a sufficiently small diameter not to come into contact with the roller paths and supported by bearings of each of the said flanges, is arranged between two successive drums;

in a kiln comprising at least two drums provided with guide sprockets, a couple of rollers rotating on their axles superimposed and provided with support bearings for the axles, the said rollers having diameters and being arranged at levels such that they roll on the periphery of the drums, are arranged between two successive drums;

at least one of the rollers of the couple is provided with cylindrical surfaces ensuring the rolling of the products having a circular cross-section to be treated;

in the zone having the highest level of temperature in the kiln, the distance between the hearth and the roller paths, measured perpendicularly to the hearth, is greater than the difference in radius between the flanges and the sprockets and in the other zones of the kiln, that distance is less than that difference;

a distance piece comprising a roller mounted on an axle borne by two flanges rolling on the hearth by means of sprockets is arranged between two neighbouring drums;

another tunnel kiln according to the invention is characterized in that the cylindrical surfaces are distributed into one or several superimposed layers and they are fast with a structure constituted by a series of adjacent distance pieces.

That other tunnel kiln comprises, moreover, preferably one or the other of the following characteristics:

the distance pieces separate the cylindrical surfaces of the lower layer and slide by a stand in at least two grooves in the hearth parallel to the axis of the kiln;

the distance pieces are provided with cylindrical recesses through which they rest on at least one of the cylindrical surfacesv A tunnel kiln applying the invention and several variants thereof are described herebelow by way of examples with reference to the figures of the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I shows a perspective view of a firing support drum.

FIG. 2 shows an end view, with a partial cutaway portion. of the drum in FIG. 1.

FIG. 3 shows the detail of a cutaway view through the axis of a cylindrical tube of the drum, with a ceramic tube to be fired arranged inside the first.

FIG. 4 shows diagrammatically a longitudinal cutaway view of the tunnel kiln.

FIG. 5 shows a cutaway view of FIG. 4 through the plane AA perpendicular to the longitudinal axis.

FIG. 6 shows a cutaway view of the same kiln through the plane BB of the same FIG. 4.

FIG. 7 shows a lateral elevation view of two adjacent drums connected together by a circular distance piece.

FIG. 8 shows a (partial) plan view of the same drums and circular distance piece with a cutaway view of a sprocket of the distance piece.

FIG. 9 shows an elevation view of two adjacent drums connected together by a distance piece having flanges and lateral sprockets and FIG. 10 is a (partial) plan view thereof.

FIG. 11 shows an elevation view of two adjacent drums and a distance piece having horizontal axles and vertical cross-members.

FIG. 12 shows a plan view of the same drums and distance piece, with a cutaway view of the cross-members at the level of the upper axle.

FIG. 13 shows an elevation view of two adjacent drums and a distance piece with a roller. moving on the hearth by means of sprockets.

FIG. 14 shows an elevation view of a system of distance pieces for support tubes distributed into two layers on the hearth of a tunnel kiln.

FIG. 15 shows a transversal cutaway view through the axis AA in FIG. 14.

FIG. 16 shows distance pieces having a cylindrical recess. each for a support tube.

FIG. 17 shows a distance piece having cylindrical recesses for two adjacent support tubes.

FIG. 18 shows two distance pieces having cylindrical recesses for support tubes distributed into two layers.

DESCRIPTION OF PREFERRED EMBODIMENTS The firing support drum 14 shown in FIGS. 1 to 3 is constituted essentially by two circular flanges I, crossed at their middle by an axle 2 provided at each end with roller sprockets 3 fixed to the axle 2 by means of keys 4.

These flanges l are drilled with a series of regularly spaced holes in which tubes 5 pass. These tubes 5 have the elongated rectilinear parts 6 to be fired (alumina tubes. in the case shown) inserted into them; stoppers 7 hold the parts 6 to be tired in placed during the moving in the kiln. All the component parts of the drum are made of refractory materials. Graphite has been used to great advantage.

The kiln according to the FIGS. 4 to 6 comprises three essential parts: a kiln body proper. 8, an inlet chamber 9, an outlet chamber 10.

Charging is effected by means of a truck 11 moving on slide rails 12 and comprising two bearing plates 13 on which a drum l4 rests by its sprockets 3. A roller 15 for intermediate contact between drums is arranged beside and rests on the bearing plates 13 by means of its trunnions 16. The loaded truck is inserted in the chamber until it is in a position such that the drum 14 and the roller 15 may be taken up by a mobile table 17 which lifts them slightly, enabling the truck 11 to release itself and to be able to be brought back to its original loading position. After this. the door 17 A of the chamber is closed. A hood 18 constituting. which the plate of the table 17, a sealed enclosure. is lowered onto the table supporting the drum 3 and the roller 15 and a vacuum is set up in that enclosure. The atmosphere of the chamber is swept by a gas current having the same composition as the atmosphere prevailing in the kiln. The hood 18 is raised. the door 18 A of the chamber leading to the kiln is opened. a truck 19 identical to the truck 11 moving on slide rails 20 extending the slide rails 12. comes and places its bearing plates 21 under the sprockets 3 and the trunnions 16 of the drum 14 and of the roller 15. The table 17 is lowered; the truck 19 moves towards the kiln with its load. the door 18 A of the chamber is closed again and the operations may be repeated.

The locking. the controls of the chamber door. the truck controls. mobile table controls. mobile hood controls, the setting up of a vacuum, the sweeping. produced by any appropriate known means. not being, in themselves, an object of the invention. have not been specially illustrated or described.

The body of the kiln proper 8, is constituted by a tunnel made of a refractory material. comprising a hearth 22 and two roller paths 23 provided for the sprockets 3 and the trunnions 16 of the drums I4 and the roller 15. These roller paths 23 are connected to the bearing plates 21 of the truck 19. The roller paths 23 are rectilinear. The upper surface of the hearth is constituted by various planes whose distance from the roller paths varies along the kiln so that the drums 14 move by rolling either on the hearth 22 through their flanges l, or on the roller paths 23 through the sprockets 3 and, consequently, for a constant moving speed. with an angular rotation speed inversely proportional to the ratio of the diameters of the sprockets and flanges.

The kiln is heated by Joule effect by means of graphite resistors whose spacing out and characteristics are suitable to the temperature curve which it is required to obtain in the kiln.

In the case of very severe conditions of thermal insulation and removal of impurities. these conditions are met by the use of graphite both for the thermal insulation and for the constituting. The full heat part of the kiln is insulated by means of graphite felt. The furnace is supported in a sealed casing 25 ensuring the preserving of the inside atmosphere.

The layer of drums constituting the load of the furnace moves at a constant linear movement speed under the effect of gravity in proportion to the slope of the kiln and in proportion to the displacing movement of a device 26 for supporting the load. not shown in detail.

for example a device with a threaded rod and a stop forming a nut on the rod, driven by a motor.

The movement by linear displacement of the layer of drums rolling on the hearth or the roller paths requires, for that movement to be effected without any perturbances, that any sliding on generating lines in contact by excluded. It is in order to do this that, according to the invention, a roller provided with trunnions 16 which are angularly independent from the roller 15 is inserted between two neighbouring drums. The rotating of two neighbouring drums 14 to which are imparted equal angular speeds results in equal tangential speeds on the generating lines in contact of the drums and of the roller. That tangential speed of the roller is independent from the speed of the trunnion 16. That sliding of the contact surfaces of drums and of rollers or of their sprockets cannot however be avoided in the intermediate zones where different angular speeds are imparted to two neighbouring drums. This, however, only constitutes a secondary phenomenon which is of no great importance, due to the small space and time in which it is localized.

In order to prevent the roller 15 from being driven out of its position between two drums by the compression efforts which are exerted in the layer, a different diameter from that of the sprocket 3 is imparted to the trunnion 16, so that the contact generating lines be situated below the line of the centres of the drums.

Instead of ensuring the spacing apart of the drums by rollers 15 inserted between flanges 1, distance pieces or spacer assemblies arranged between sprockets 3, may be used. Thus, (FIGS. 7 and 8), it is possible to use a circular distance piece or spacer assembly 40 arranged on an axle 41 borne by two sprockets 42. Likewise, a distance piece or spacer assembly may be formed (FIGS. 9 and 10) even by assembling two flanges 50 on two axles 51 borne by two sprockets 52, three sprockets 53 borne by axles 54 also being assembled on the flanges 50, so that the centres of the axles 54 by slightly below the centres of the axles 51, the diameters of the sprockets 53 being such that these latter do not bear against the roller path 23. Care should be taken to see that the sum of the number of the sprockets 52 and 53 be odd (five, seven, etc...).

A distance piece or spacer assembly may also be con stituted (FIGS. 11 and 12) by arranging, between two drums, two rollers 60, 61 rotating on two axles 62 connected together by two cross-pieces 63 drilled with holes 64. One of the rollers 60, 61 could, moreover, be constituted by a drum.

It is also possible to constitute a distance piece or spacer assembly (FIG. 13) between two adjacent drums by a roller 73 mounted on an axle 74 borne by two flanges 75 moving on the hearth 22 by means of sprockets 76.

In the central full heat part of the kiln 8, the distance between hearth and roller surface is greater than the difference in radius between the flanges l and the sprockets 3. The drums 14 rolling on the paths 23 will therefore have a greater rotating speed than in the other parts of the kiln where they roll on the hearth, this taking into account the great thermochemical differential stresses on the products to be fired in that zone of the kiln as a function of its position, these stresses being equalized by making the object rotate on its own axis at a higher speed.

At the end of the kiln 8, a zone 27 is provided in which the drums are subjected to a more intense cooling, by constituting the tunnel with metallic casings 28 through which a forced flow of cool gas circulates (circuit of the arrows, FIG. 6).

In the case where no drums are used, the drive of the support tubes may be provided by maintaining these latter spaced apart from one another by means of distance pieces or spacer assemblies such as those illustrated in FIGS. 14 to 18.

On the inclined hearth 22 of the kiln in FIGS. 14 and 15, support tubes 82 are arranged in two superimposed layers. Inside the tubes are placed the products 83 to be fired. In the hearth of the kiln are formed two recesses 84 in which are arranged parts 85 comprising a stand 86 having a rectangular cross-section. each provided with a cleat 87. The parts 85 have, for their length, the distance between centres required between two neighbouring tubes 82a and 82b. A push rod, not shown, makes the row of these parts 85 advance by a length equal to a distance between centres at the occasion of each loading of tubes 82c, 82d. The thrust is transmitted by the column of the stands 86. No stress is practically imposed on the cleats 87, except by the horizontal component of the weight of two loading tubes, the friction of the sliding of a tube 82 on the cleats 87 is therefore very limited.

Instead of moving the distance pieces or spacer assemblies 85 in grooves 84, it is possible to produce them in the form of rectangular parts 88 provided with a recess 89 which can be placed on each tube 82 (FIG. 16) or in the form of parts 90 provided with recesses 91 which may be placed on two or several neighbouring tubes (FIG. 17).

These parts 90 may also be placed on the tubes of the second layer of a load having two layers (FIG. 18) or on the third layer of a load having three layers.

Although the embodiments of the tunnel kiln which have just been described appear preferable, it will be understood that various modifications may be made thereto without going beyond the scope of the invention, it being possible to replace certain elements of the kiln by others which would fulfill the same or a like technical function therein. More particularly, the heating of the kiln may be ensured by fuel burners.

What is claimed is:

1. In a tunnel kiln for the heat treatment of products having a circular cross-section by moving them along a longitudinal axis of the kiln, the improvement comprising: a drum mounted for rotation about its axis and for progressing through said kiln by rolling along the longitudinal axis thereof, and means for supporting said products within said drum for rotation about their axis at right angles to the longitudinal axis of the kiln and for simultaneously travelling in a circular path about the axis of said drum and at right angles to the longitu dinal axis of said kiln.

2. The tunnel kiln according to claim 1, wherein: said kiln comprises means defining two laterally spaced generally horizontal roller paths extending along the longitudinal axis thereof, and wherein said drum is provided, at each of its ends, with a roller sprocket riding on a respective path and having a diameter different from that of said drum.

3. The tunnel kiln according to claim 1, wherein: said kiln is provided with a horizontal hearth and said drum is provided with at least two circular flanges on the drum periphery which rest on the hearth for mounting the drum for rotation about its axis and for rolling progression through said kiln.

4. The tunnel kiln according to claim 2, wherein: said drums are at least two in number, a spacer assembly is mounted on each roller path defining means and each spacer assembly comprises a roller in frictional contact with the drum and mounted for idling rotation about its axle between two successive drums and having guide trunnions on respective sides thereof rolling on said laterally spaced roller paths respectively.

5. The tunnel kiln according to claim 4, wherein: the diameter of the guide trunnions of the spacer assembly roller is smaller than that of the sprockets for said drums riding on said roller paths.

6. The tunnel kiln according to claim 2, wherein: said drums number at least two and wherein said drums are spaced by means of a circular spacer assembly including circular pieces mounted for rotation about their axis and being in peripheral contact with respective roller sprockets for said drum on said roller path, each spacer assembly being further provided with guide sprockets mounted for idling on the axle connecting said circular pieces on respective sides of said kiln, said guide sprockets arranged between two successive drums.

7. The tunnel kiln according to claim 2, wherein: said drums are at least two in number, spacer assemblies are provided on respective paths on opposite sides of said drum, each spacer assembly consisting of a flange an even number of flange mounting sprockets having the same diameter and rolling on the same roller paths as the drum sprocket rollers supporting said flange and an odd number of idler sprockets of a number greater than the even number of sprockets being mounted for rotation on said flange and having a diameter sufficiently small so as not to come into contact with the roller paths but bearing upon said even number sprockets for transmitting motion therebetween with said flanges being arranged between two successive drums.

8. The tunnel kiln according to claim 2, wherein: said drums are at least two in number and two successive drums are separated by paired rollers rotating on two superimposed axles on opposite sides of said path with said rollers being in rolling contact with the periphery of adjacent drums.

9. The tunnel kiln according to claim 8, wherein: at least one of the rollers of the couple is in frictional rolling contact with at least one of said drums to insure the rotation of the products having a circular cross-section about their axis during rotation of said at least one drum.

10. The tunnel kiln according to claim I, wherein: the drums are at least two in number, and a spacer assembly is arranged between two adjacent drums, said spacer assembly including a roller in frictional contact with the peripheries of respective drums and mounted on an axle for rotation about a horizontal axis, borne by flanges on opposite ends thereof, and wherein said tunnel kiln includes a horizontal hearth with sprockets mounted for rotation at the bottom of said flanges and rolling on said hearth along with the peripheries of said drums.

11. In a tunnel kiln for heat treatment of products having a circular cross-section, comprising means for making the products rotate in a circular path within the kiln perpendicular to their longitudinal axis, means for moving them along the longitudinal axis of the kiln, the means for subjecting them at the same time to a rotation about that longitudinal axis, by rolling on cylindrical surfaces which support the product, the improvement comprising: means forming superimposed layers of cylindrical surfaces and spacer assemblies for mechanically linking said cylindrical surfaces longitudinally,

12. The tunnel kiln according to claim 11, wherein: said kiln includes a horizontally inclined hearth, and wherein the spacer assemblies which separate the cylin drical surfaces comprise slide stands mounted in laterally spaced grooves within the hearth which extend parallel to the axis of the kiln.

13. The tunnel kiln according to claim 11, wherein: the spacer assemblies are formed of rectangular parts provided with cylindrical recesses which rest on respective adjacent cylindrical surfaces. 

1. In a tunnel kiln for the heat treatment of products having a circular cross-section by moving them along a longitudinal axis of the kiln, the improvement comprising: a drum mounted for rotation about its axis and for progressing through said kiln by rolling along the longitudinal axis thereof, and means for supporting said products within said drum for rotation about their axis at right angles to the longitudinal axis of the kiln and for simultaneously travelling in a circular path about the axis of said drum and at right angles to the longitudinal axis of said kiln.
 1. In a tunnel kiln for the heat treatment of products having a circular cross-section by moving them along a longitudinal axis of the kiln, the improvement comprising: a drum mounted for rotation about its axis and for progressing through said kiln by rolling along the longitudinal axis thereof, and means for supporting said products within said drum for rotation about their axis at right angles to the longitudinal axis of the kiln and for simultaneously travelling in a circular path about the axis of said drum and at right angles to the longitudinal axis of said kiln.
 2. The tunnel kiln according to claim 1, wherein: said kiln comprises means defining two laterally spaced generally horizontal roller paths extending along the longitudinal axis thereof, and wherein said drum is provided, at each of its ends, with a roller sprocket riding on a respective path and having a diameter different from that of said drum.
 3. The tunnel kiln according to claim 1, wherein: said kiln is provided with a horizontal hearth and said drum is provided with at least two circular flanges on the drum periphery which rest on the hearth for mounting the drum for rotation about its axis and for rolling progression through said kiln.
 4. The tunnel kiln according to claim 2, wherein: said drums are at least two in number, a spacer assembly is mounted on each roller path defining means and each spacer assembly comprises a roller in frictional contact with the drum and mounted for idling rotation about its axle between two successive drums and having guide trunnions on respective sides thereof rolling on said laterally spaced roller paths respectively.
 5. The tunnel kiln according to claim 4, wherein: the diameter of the guide trunnions of the spacer assembly roller is smaller than that of the sprockets for said drums riding on said roller paths.
 6. The tunnel kiln according to claim 2, wherein: said drums number at least two and wherein said drums are spaced by means of a circular spacer assembly including circular pieces mounted for rotation about their axis and being in peripheral contact with respective roller sprockets for said drum on said roller path, each spacer assembly being further provided with guide sprockets mounted for idling on the axle connecting said circular pieces on respective sides of said kiln, said guide sprockets arranged between two successive drums.
 7. The tunnel kiln according to claim 2, wherein: said drums are at least two in number, spacer assemblies are provided on respective paths on opposite sides of said drum, each spacer assembly consisting of a flange an even number of flange mounting sprockets having the same diameter and rolling on the same roller paths as the drum sprocket rollers supporting said flange and an odd number of idler sprockets of a number greater than the even number of sprockets being mounted for rotation on said flange and having a diameter sufficiently small so as not to come into contact with the roller paths but bearing upon said even number sprockets for transmitting motion therebetween with said flanges being arranged between two successive drums.
 8. The tunnel kiln according to claim 2, wherein: said drums are at least two in number and two successive drums are separated by paired rollers rotating on two superimposed axles on opposite sides of said path with said rollers being in rolling contact with the periphery of adjacent drums.
 9. The tunnel kiln according to claim 8, wherein: at least one of the rollers of the couple is in frictional rolling contact with at least one of said drums to insure the rotation of the products having a circular cross-section about their axis during rotation of said at least one drum.
 10. The tunnel kiln according to claim 1, wherein: the drums are at least two in number, and a spacer assembly is arranged between two adjacent drums, said spacer assembly including a roller in frictional contact with the peripheries of respective drums and mounted on an axle for rotation about a horizontal axis, borne by flanges on opposite ends thereof, and wherein said tunnel kiln includes a horizontaL hearth with sprockets mounted for rotation at the bottom of said flanges and rolling on said hearth along with the peripheries of said drums.
 11. In a tunnel kiln for heat treatment of products having a circular cross-section, comprising means for making the products rotate in a circular path within the kiln perpendicular to their longitudinal axis, means for moving them along the longitudinal axis of the kiln, the means for subjecting them at the same time to a rotation about that longitudinal axis, by rolling on cylindrical surfaces which support the product, the improvement comprising: means forming superimposed layers of cylindrical surfaces and spacer assemblies for mechanically linking said cylindrical surfaces longitudinally.
 12. The tunnel kiln according to claim 11, wherein: said kiln includes a horizontally inclined hearth, and wherein the spacer assemblies which separate the cylindrical surfaces comprise slide stands mounted in laterally spaced grooves within the hearth which extend parallel to the axis of the kiln. 